will a sim card from an iphone 4 work in an iphone 5?

No, the iPhone 4 used the micro sim, the iPhone 5 the nano sim.

http://support.apple.com/kb/HT5163

iPhone 4 uses a Micro sim, iPhone 5 uses a Nano sim. You'd have to cut the Micro sim down...best to just get a new sim.

YES you can you would need to cut your old sim to make it fit

You need to place the new SIM card on top of the old and cut like that

Just be careful not to cut too much

IT WORKED FOR ME Good Luck